A T1 digital transmission system is a publicly known telephony system in which a plurality of pulse coded modulation (PCM) voice signals are transmitted by a single digital transmission link using time division multiplexing. Along with the voice signals, signalling data known as the A-bit and B-bit, are also transmitted to provide signalling information (such as on-hook, off-hook, dialing digits, and call progress information). T1 transmission normally uses a technique called "robbed bit" signalling which allows the signalling data to be multiplexed onto the transmission link along with the voice signals. Disadvantageously, robbed bit signalling degrades the fidelity of the voice channels, makes tandem links difficult to implement, and poses other significant problems with adaptive pulse code modulation (ADPCM) links.
There are two framing formats commonly used for T1 transmission: an older D4 format, and a newer extended superframe format (ESF). Since both formats are similar in so far as signalling is concerned, only the D4 format is discussed herein. In D4 framing, 8000 frames per second are transmitted. Each frame has 193 sequential bits including one framing bit and 192 voice bits. The voice bits include an 8 bit PCM sample from each of twenty four channels. Thus, twenty four samples are time division multiplexed (TDM). Each 8 bit PCM sample is obtained by sampling a voice signal at an KHZ sampling rate to produce a 64K bps (8 bit sample and 8 Khz sampling rate) signal that is TDM with 23 other TDM channels to produce a resultant digital transmission rate of 1.544 Mbps.
The frames are transmitted in sequential groups of twelve frames. Each group in known as a multiframe. Signalling information is conveyed in each channel in the least significant bit (bit 8) of frames 6 and 12 of a 12-frame multiframe. Such signalling method is known as the "robbed bit" method because the signalling bits displace the original least significant bit of the voice signal in frames 6 and 12. That causes a degradation in the quality of the voice signal. Instead of the original 8 bit dynamic range for which the voice signal was originally quantized, the dynamic range is reduced to seven-and-a-fraction bits with a resultant increase in quantization noise and harmonic distortion.
Another disadvantage of robbed bit signalling occurs when a telephone call spans three or more telephone switches referred to as a tandem connection. Consider a call which originates on switch A, connects to switch B, and finally terminates at switch C. Switches A and C are generally not synchronized as far as multiframes are concerned, and consequently, switch C cannot easily recover the signalling bits conveyed in frames 6 and 12. In fact, a great deal of hardware and/or software complexity is required to convey the signalling bits on a tandem connected communications link.
ADPCM introduces a further problem with robbed bit signalling. In ADPCM, two audio channels are compressed to fit into the bandwidth of a single PCM audio channel. Each of the original audio channels (8 bit samples) effectively occupy a 4-bit sample. When robbed bit signalling is introduced onto ADPCM links, the resulting degradation in dynamic range and voice fidelity is even more severe than with conventional PCM. Another approach to ADPCM alleviates the degradation in voice quality by combining the signalling bits of each channel onto a common channel. However, this means that one of the available channels is now dedicated to carrying signalling information and therefore cannot be used as a voice channel. Consequently, a voice channel is effectively lost or eliminated.